Switch arrangements are used widely in automotive applications. For example, for power switches for driving injector or braking valves (e.g. in ABS systems) and for automotive power supplies. Switch arrangements are also used in automotive activation systems such as an igniter system for airbag deployment and a seat belt pretensioner activation system.
An igniter system comprises an activation element or igniter element which converts electrical energy to heat. Typically, the igniter element, also known as a squib, comprises a hot wire bridge which is heated by a firing signal, for example a firing current of 1-2 Amps (A). In, for example, airbag applications, the heat generated in the igniter element ignites a pyrotechnic material adjacent the igniter element which burns a propellant. This produces gas to inflate the airbag.
A particular concern for automotive manufacturers is the possibility of activation elements activating inadvertently due to a fault. For example, inadvertent activation of an airbag may disturb a driver and possibly cause an accident. Thus, drive circuits used for generating the firing or activation signal are designed to minimise inadvertent activation and to ensure reliable operation. FIG. 1 illustrates a known simplified airbag activation circuit.
FIG. 1 illustrates an igniter element or squib 101 coupled to a drive circuit 103. The drive circuit 103 is implemented in a single Application Specific Integrated Circuit (ASIC) and comprises functionality for generating the firing signal which activates the squib 101. More specifically, the drive circuit 103 comprises a switch arrangement including a high side switch FET (Field Effect Transistor) 105 and a low side switch FET 107. During normal operation, when the airbag is not deployed, the high side FET 105 and the low side FET 107 are both in an off state and no current can flow through the squib. The use of two switch transistors in series provides increased reliability and failure prevention. Particularly, if either one of the switch FETs short circuits, this will not result in an activation of the airbag as the other switch FET will be in the off state.
The high side FET 105 is controlled by a high side control circuit 109 and the low side FET 107 is controlled by a low side control circuit 111. Both control circuits 109 and 111 are coupled to a microprocessor 110 which is connected to one or more crash sensors (not shown), such as an accelerometer, to determine when a particular crash condition is occurring in which an airbag should be deployed. The low side control circuit 111 produces a signal which switches the low side FET 107 off during normal operation and on if the airbag is being activated. The high side control circuit 109 also controls the high side FET 105 to be off during normal operation and on during airbag activation.
For current saving purposes, the high side control circuit 109 is further arranged to control the operation of the high side FET 105 such that it operates in a current limitation mode to limit the current to the squib 101 to the required value, which is typically 1.2 A.
Typically, the same energy supply is used for a plurality of airbags and the current limitation prevents that this energy supply is used up by a short circuit in one airbag. For example, during a crash, the upper squib end may be short circuited to ground. If the current through the high side FET 105 is not limited, the resulting current would become exceedingly high thereby quickly draining the energy supply and possibly preventing the activation of other airbags.
Typically, the drive circuit 103 is not directly connected to the energy supply. Rather, a power switch transistor known as a safing switch 113 is coupled in series with the drive circuit 103. The safing switch 113 is generally an external discrete FET component. The safing switch 113 provides further failure prevention by providing additional redundancy in the airbag activation operation.
Specifically the operation of the safing switch 113 is controlled by a control circuit 115 in response to different sensor inputs than those used for activating the drive circuit. Typically the safing switch 113 is controlled by a completely different microprocessor operating a different crash detection algorithm and with different sensor inputs than for the drive circuit. Thus, the airbag is only activated if both redundant evaluations detect the occurrence of a crash in which case the high side FET 105 and the low side FET 107 of the drive circuit as well as the safing switch 113 are switched on. The safing switch 113 is operated as a simple on/off switch. In some applications, several safing switches are used to provide independent safety switches for different drive circuits. For example, each squib may be provided with its own safing switch.
The safing switch 113 is coupled to a reverse flow blocking diode 117. The reverse flow blocking diode 117 is connected to a capacitor 125 coupled to receive the battery voltage Vbat and which provides the power supply to the drive circuit 103 and squib 101. The capacitor 125 ensures that energy may be provided to the airbag activation system even if the connection to the battery is broken during the crash. However, as the capacitor 125 may be discharged, for example after the car has been switched off for a given duration, an electrical path exists from the upper end of the squib to ground through the capacitor 125 and the parasitic diodes 119, 121, which parasitic diodes are inherent features of FETs. Accordingly, in the absence of the blocking diode 117, a short circuit resulting in a voltage being applied to the lower end of the squib would result in a current flowing through the squib and thereby activating the airbag. The blocking diode effectively breaks this path. The blocking diode may typically be common to a plurality of drive circuits.
A number of disadvantages are associated with the prior art arrangement of FIG. 1.
Firstly, the requirement for an external safing FET tends to increase the cost and complexity of the arrangement. Furthermore, the safing FET tends to be relatively bulky and as the FET is external to the drive circuit, it requires additional operations during manufacturing.
Furthermore, the prior art arrangement results in a significant energy dissipation in the high side FET 105 which accordingly must be relatively large.
Specifically, the energy stored in the reservoir capacitor is given by
  E  =            1      2        ⁢          C      ·              V        2            where C is the capacitance of the capacitor and V is the voltage across the capacitor. Hence, in order to store sufficient energy to ensure that the squib is activated, while maintaining the size and cost of the capacitor acceptably low, it is required that the capacitor is charged to a relatively high voltage. Typically, the capacitor is charged to a voltage of around 35-36 volts (V).
During activation, the low side FET 107 is fully switched on resulting in a typical voltage drop of less than 2V. Furthermore, the impedance of the squib 101 is relatively low resulting in a typical voltage drop of less than 2V. The voltage drop over the blocking diode 117 is typically around 1V. Furthermore, the safing FET 113 is fully switched on during activation resulting in a typical voltage drop of around 1 V (the on resistance of the safing FET 113 is typically lower than that of the low side FET 107). Accordingly, during the current limiting operation of the high side FET 105, the voltage drop from drain to source is typically in the order of 30V. Typically the current is limited to around 1.2 A and the squib is fired in typically 1-2 ms. Therefore, the energy dissipation in the high side FET 105 during activation is around 30V×1.2 A×·2 ms=72 milli-Joules (mJ). This energy needs to be absorbed by the high side FET 105 without resulting in a thermal shutdown of the FET. The size of a FET is driven by the amount of energy to be dissipated: the higher the energy, the larger the size of the FET. Thus, in order to meet this energy requirement, it is necessary that the high side FET 105 is physically large.
However the requirement for a large FET has significant impact on the ASIC cost. Furthermore, as the required size depends on the energy absorption requirement, the design cannot take full advantage of the advances in manufacturing technology. For example, as improvements in lithography processing are achieved, smaller transistors can be formed resulting in smaller areas being required to implement circuits. This allows for higher integration and may allow more circuitry to be included in the same ASIC. However, the high side FET cannot be shrunk due to the fact that in operation it is required to dissipate 120 mJ of energy.
There is therefore a need to provide an improved switch arrangement which when used in an activation system mitigates the above problems and disadvantages.